Icosahedral virus assembly

Complex viruses Some virions are even more complex, being composed of several separate parts, with separate shapes and symmetries. The most complicated viruses in terms of structure are some of the bacterial viruses, which possess not only icosahedral heads but helical tails. In some bacterial viruses, such as the T4 virus of Escherichia coli, the tail itself is a complex structure. For instance, T4 has almost 20 separate proteins in the tail, and the T4 head has several more proteins. In such complex viruses, assembly is also complex. For instance, in T4 the complete tail is formed as a subassembly, and then the tail is added to the DNA-containing head. Finally, tail fibers formed from another protein are added to make the mature, infectious virus particle. 

The Hu and Bentley model is the only one that tries to describe VLP production and assembling in baculovirus infected insect cells [105]. Nevertheless, regarding VLP assembly, the formalism presented is completely theoretical and based on the assembly pathway of icosahedral viruses. From a process development point of view, this model does not generate enough output to make it applicable to bioreaction operational parameters definition. However, it can be used as the basis for a more structured approach to the VLP assembling process in baculovirus infected insect cells. 

Rhinoviruses belong to the picornaviridae family small icosahedral viruses with an average diameter of 300 A and a molecular mass of approximately 8.5 X 10 Da. Like all picornaviruses, HRVs are made of a protein capsid that encases a single-stranded, positive-sense RNA molecule of about 7000 bases. The capsid is built from 60 copies of viral proteins 1, 2, 3 and 4 (VPl, VP2, VP3, and VP4). VPl, VP2 and VP3 assemble on the exterior to form the protein shell, and VP4 resides in the interior of the capsid,... 

The CPMV is also an RNA-containing plane virus. The icosahedral virus particle is formed from 60 copies of two different types of protein subunits (large 37 kDa and small 23 kDa). These proteins assemble into a pseudo (7 = I) particle approximately 30 nm in diameter. Very high... 

Fig. 1 Assembly of T = 3 icosahedral virus particle showing the three subunits of the asymmetric unit Al, BI, Cl. (Used with permission from http //mmtsb.scripps.edu/viper/viper.htm)...

Figure 2 Principal strategies for the formation of well-defineci inorganic nanostmctures using viral capsids. For simplicity, each method is shown for an icosahedral virus. (a,b) Metal ions or clusters can bind electrostatically to either (a) the inside, or (b) the outside of virus capsids. The high local concentration of inorganic material can serve as a nucleation point for (a) a nanoparticle constrained by the capsid dimensions or (b) a shell covering the exterior capsid surface, (c) Capsid monomers can bind directly to a nanoparticle surface and use it as a nucleation site for assembling an intact capsid, (d) DNA complementary to the viral RNA can be attached to a nanoparticle. Binding the native capsid RNA then allows nucleation of the protein monomers and their subsequent assembly into a full capsid.

Figure 30 Quasi-equivalency of natural and synthetic supramolecular systems with cylindrical and spherical shapes (a) TMV, (b) icosahedral virus synthetic analog of (a) and (b) have been self-assembled from (c) tapered, and (d) conical monodendrons. (Courtesy of Nature 1998, 391, 8, 161. Copyright 1998 Macmillan Magazines Limited.)...

There are also more complex virus assemblies where the capsid is neither purely helical, nor purely icosahedral, presenting a combination of icosahedral and heUcal shape. Extra features may be present, like a complex outer wall or a head-tail morphology. Examples of viruses with a complex structure are (i) the variola virus (which causes smallpox) presenting a unique outer wall and capsid (Fig. 15.1C) and (ii) bacteriophages, which (most of them) present a head-tail morphology structure—this is unique to viruses that only infect bacteria (Fig. 15.1D). 

Plenary 4. George J Thomas Jr et at, e-mail address thomasgj ,cctr.mnkc.edu (RS). Protein folding and assembly into superstructures. (Slow) time resolved RS probing of virus construction via protein assembly into an icosahedral (capsid) shell. 

The capsids of polyoma virus and the related SV40 have icosahedral symmetry, with 72 pentameric assemblies of the major capsid protein. The pentamers are linked to their neighbors by flexible arms, with a p strand that augments a p sheet in the invaded pentamer. These flexible arms allow the pentamers to be linked together with both fivefold and sixfold symmetry. 

The non-enveloped human viruses all have icosahedral capsids. The structural proteins undergo a self-assembly process to form capsids into which the viral nucleic acid is packaged. Most non-enveloped viruses accumulate within the cytoplasm or nucleus and are only released when the cell lyses. 

Sixty copies of the latter are assembled around the RNA in an icosahedral array (Fig. 7-14) to form the virion. The structure of the similar satellite tobacco mosaic virus has also been described in detail.486 487... 

Raman difference spectroscopy has also been used to understand the molecular mechanism of viral core assemblies yielding information on viral subunits from precursor and mature states. Benevides et al. employed Raman difference spectroscopy to investigate conformational changes of the protein building blocks of the icosahedral core of a double-stranded RNA (p6 virus during viral procapsid and capsid assembly [18],... 

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